1. Field of the Invention
This invention relates to an apparatus for optically detecting information signals which are recorded on a record carrier, and more particularly an optical information signal detecting apparatus suitable for a optical video disc player with an optical system wherein light beams used for reading the information signals and for a tracking servo-control or a focus servo-control, respectively, are incident on a signal record surface of a record carrier having a signal record track formed thereon, and the detection of the recorded signals and the tracking servo-control or the focus servo-control are effected by the use of reflected light beams from the signal record surface.
2. Description of the Prior Art
In the conventional optical video disc, the center of rotation of a master disc during a signal recording operation or a cutting operation does not usually accord with the center of rotation of a video disc manufactured from the master disc for use in signal reproduction during a reproducing operation or a reading or playback operation for recorded signals. For this reason, when the video disc used for the signal reproduction is rotated to read or playback the recorded signals in a video disc player, its movement is somewhat eccentric and, consequently, each given portion of a record track to be read appears to move in a complex path rather than a simple circle. Such movement results in a displacement of the record track from a predetermined position, so that the recorded information signals cannot be accurately read. In order to compensate for the eccentric movement of the disc, it has been proposed in the conventional video disc player to provide a time base correction (TBC) mirror disposed in an optical path of a light beam passing through an objective lens so as to operate in cooperation with a tracking control mirror.
That is to say, a focussing point or a beam spot of an incident light beam formed on the surface of the disc by the objective lens is moved in the forward direction along the record track by the TBC mirror, and is moved in the radial direction of the disc by the tracking mirror, so that the beam spot moves on the surface of the disc in both radial and tangential directions in response to the eccentric movement of the disc. As a result, the recorded information signals can be accurately read despite the eccentric movement of the record track. However, the particular positional arrangement of the TBC mirror and the tracking control mirror and the compatibility of these mirrors with a focus servo-control system has not been previously set forth. Thus it has been necessary to provide a new system in which the servo-control by the TBC mirror can be optically performed without disturbing the functions of the tracking servo-control system and the focus servo-control system.
Incident light beams on the disc comprise, in general, a beam for reading the information signals and two beams for the tracking servo-control locating at the both sides of the former beam. These three beams form the respective spots on the disc along the record track after passing through the objective lens to be reflected from the record track and then pass through the objective lens again to reach a photo-detector comprising photo-diodes or the like. The beam spots of the reflected beams formed on the photo-detector are dot-like or circular and are continually moved following the movement of the tracking mirror. Thus the output from the light-receiving surface of the photo-detector which must have relatively large surface area, does not have a uniform level. Also, the signal-to-noise ratio of reproduced signals is degraded because of the sensitivity of the photo-detector to stray light leaking from the outside.
The light beams for reading the information signals, such as the tracking servo-control and the focus servo-control are usually obtained from a single light source, such as a laser device. These beams may be separated respectively from a single light beam. In conventional apparatus, such separation is performed by an ordinary beam splitter and the plural beams are merely spatially split. Accordingly, the respective split beams after reflection on the disc are positioned closely to each other. Such spatial split of the beam requires the increased accuracy in the position and direction of the beam splitter, and it is very difficult to arrange the apparatus as a compact unit. Considerable stray light from the beams is caused when the single beam is split into the plural beams by the beam splitter and there is a concomitant loss in the intensity of the light beam.